The present invention relates generally to ultrasonic measurements of biological tissue parameters for medical diagnostics, and more particularly to a method and a device for measurements of ultrasound velocity in tissue aimed at determination of hydration and water content status of selected tissue or the entire human body.
Tissue water content, a specifically, muscle water content (MWC), is an important and informative diagnostic parameter. Dehydration decreases cognitive and physical work capabilities, while the excessive hydration (swelling, edema) is a common symptom of cardiac, hepatic or renal pathology, malnutrition and many other pathologies and diseases. Edema causes muscle aches and pains and may affect the brain, causing headaches and irritability. Edema is a major symptom for deep venous thrombosis. It may be caused by allergies or more serious disorders of the kidney, bladder, heart, and liver, as well as food intolerance, poor diet (high sugar & salt intake), pregnancy, abuse of laxatives, diuretics, drugs, the use of contraceptive pills, hormone replacement therapy, phlebitis, etc.
Monitoring of muscle water content can serve as an important indicator of body hydration status in athletes during the training as well as in soldiers during deployment. It is generally known that body hypohydration causes severe complications, health and performance problems. It is generally known that a body water weight loss of up to 1% causes thirst, 2%—vague discomfort and oppression, 4%—increased effort for physical work, 5%—difficulty concentrating, 6%—impairment in exercise temperature regulation, increases in pulse and respiratory rate; 10%—spastic muscles; 15%—death. Soldiers commonly dehydrate 2–5% of body weight due to high rate of water loss from environmental exposure and performing stressful physical work. Dehydration by modest amounts (2%) decreases cognitive and physical work capabilities, while larger water losses have devastating effects on performance and health. Numerous pathologic signs and symptoms due to body dehydration include digestion problems, high blood pressure, muscle cramps, etc. MWC monitoring by an objective instrument may help prevent hazard thresholds. This is important because subjective indicators like thirst can be inadequate.
Control of MWC in athletes and soldiers could help in monitoring total body hydration during long-term endurance exercise or performance in hot weather conditions.
There are several methods for assessing total body water, as the most prominent indicator of hydration status. Most of these methods are based on bioelectrical impedance and conductance methods. U.S. Pat. No. 4,008,712 issued to Nyboer discloses method and apparatus for performing electrical measurement of body electrical impedances to determine changes in total body water in normal and deranged states of the body, U.S. Pat. No. 5,615,689 issued to Kotler discloses a method of predicting body cell mass using impedance analysis, U.S. Pat. No. 6,280,396 issued to Clark discloses an apparatus and method for measuring subject's total body water content by measuring the impedance of the body, and U.S. Pat. No. 6,459,930 issued to Takehara et al. discloses a dehydration condition judging apparatus by measuring bioelectric impedance.
The aqueous tissues of the body, due to their dissolved electrolytes, are the major conductors of an electrical current, whereas body fat and bone have relatively poor conductance properties. Significant technical problems eliminated the viability of many electrical methods for in vivo body composition analyses. Oversimplifications in formulae in the standard biological impedance analysis methods lead to problems. There is a more complex approach, based on measuring resistance and reactance over a wide range of frequencies. The technique based on this approach is called bioelectrical impedance spectroscopy. U.S. Pat. No. 6,125,297 issued to Siconolfi discloses a method and apparatus for determining volumes of body fluids in a subject using bioelectrical response spectroscopy.
Regardless of the choice of single or multifrequency method, the impedance index alone is not an accurate predictor. Additional anthropometric terms (i.e., weight, age, gender, race, shoulder width, girth, waist-to-hip ratio, body mass index) need to be included in the prediction model to reduce the standard error of the estimate. In summary, the downside of the water content assessment methods based on the measurements of electrical properties of tissues is low accuracy, significant dependence of testing results on the anthropometrical features of the subject and on electrolyte balance.
There are existing much more precise laboratory methods of assessment of body water content such as Magnetic Resonance Imaging and various so called Dilution methods, but these methods use bulky equipment, are lengthy and laborious to perform and are impractical for most applications. The basic principle of the dilution techniques for body water content is that the volume of a compartment can be defined as the ratio of the dose of a tracer, administered orally or intravenously, to its concentration in that body compartment within a short time after the dose is administered. Typically, two blood (or urine) samples are collected: one just before administration of the dose, to determine the natural background levels, and the second sample after waiting a sufficient amount of time for penetration of the tracer within the compartment of interest. It is laborious and not very accurate, and is typically used only for reference with other measurement techniques.
There is therefore a need for a simple and highly accurate method and device for monitoring tissue hydration status that can be used in a broad range of field conditions.